In many manufacturing processes it is necessary to measure the surface characteristics of polished, plated, painted or otherwise finished objects. A great many measurements of surface quality may be based upon the evaluation of light reflected therefrom. Among such measurements commonly employed in manufacturing are Gloss, which is a measure of the dispersion with which light is reflected from a surface, Distinctness of Image (DOI) which is a measure of how clearly an object is reflected by the surface as well as Orange Peel and Roughness which are measurements of surface texture. There are various specific definitions accepted in the art for the foregoing terms as well as various and particular protocols for measuring or otherwise defining each of such properties, all of which can be determined in accord with the present invention by the measurement of light reflected from a surface.
There are many methods and apparatus known and available to those of skill in the art for measuring the optical characteristics of finished surfaces. In many of these methods, a beam of light is directed normal to a surface and the light reflected or scattered therefrom is measured at various angles from the normal. By determining the spatial profile of the reflected and/or scattered light, various optical properties of the surface may be determined. In other instances, the beam of light is directed at the surface at an angle of less than 90 degrees, the intensity of the reflected and/or scattered light is measured as a function of its angle and appropriate correction factors are applied to enable determination of the surface's optical properties. Such techniques are in relatively widespread use in many industries and are well-known to those of skill in the art.
Optical measurements of surface quality are valuable in a production process for quality control purposes; and obviously, it is highly desirable to fully implement such techniques in mass production processes as, for example, in the manufacture of automobiles. In order to increase efficiency of measurement and throughput of a system, it would be preferred that measurements be made "in real time," that is to say as various parts travel past a measuring station, or in some instances as a mobile measuring station scans a stationary part.
Problems are encountered in implementing such techniques because of specific geometric requirements of the optical systems. In those instances where a curved surface is being measured, care must be taken to ensure that the measuring light beam is incident to the curved surface at a precisely predefined angle. This may be readily accomplished by measuring the surface curvature and properly positioning the optical components for each point being measured. However, the time required for such repeated repositioning precludes the use of a system of this type in a continuous process. In those instances where a planar surface is being characterized, the light source and detector may be appropriately prepositioned to enable measurements to be made as the surface moves therepast; however, slight height variations, surface curvature or other irregularity will displace the reflected beam, thereby influencing the accuracy of the measurement. U.S. Pat. No. 4,139,307 discloses a surface inspection system which scans a light beam across a planar surface and spatially resolves the reflected beam to measure surface gloss. The apparatus of the '307 patent is limited in use insofar as any curvature in the surface being inspected, transverse to the direction of motion will deflect the light beam so as to prevent accurate measurement of surface quality. Likewise, U.S. Pat. No. 4,568,835 is directed to an optical inspection system for detecting contamination upon a photomask. As disclosed therein, an oblique beam of light is scanned upon a planar substrate and scattered light is detected to determine the degree of surface contamination. The particular optical system employed constricts use of the '835 invention to the inspection of strictly planar substrates.
U.S. Pat. No. 4,092,068 discloses a surface inspection sensor adapted to measure topographic features of a moving substrate. The sensor of the '068 patent illuminates a surface with a beam of light and employs a number of detectors disposed so as to receive reflected light from the surface. Variations in surface topology will cause a differential receipt of light by the various detectors and this difference can be correlated with gross surface features. The '068 patent is directed to the measurement of macroscopic features and cannot be employed for gloss, distinctness of image or other such surface measurements. Accordingly, it will be appreciated that there is a need for a system for testing the optical characteristics of surface finishes of curved, continuously moving surfaces in a production environment.
The present system provides for the rapid, economical measurement of surface characteristics of painted, plated, polished or otherwise finished surfaces and may be readily adapted for use in a continuous process and for measurement of characteristics of curved or flat surfaces. The present invention provides for a simple, economical system which repeatedly adjusts its geometry to the configurations of the surface being measured to maintain proper optical alignment. Through the use of associated data processing systems, various properties may be reliably and accurately measured. These and other advantages of the present invention will be readily apparent from the drawings, description, discussion and claims which follow.